WASHINGTON (AP) — The head of the House Oversight and Government Reform Committee has subpoenaed the co-chairman of the independent review board that investigated last year’s attack on the U.S. diplomatic mission in Benghazi, Libya, to answer questions about the panel’s findings behind closed doors.

Rep. Darrell Issa, R-Calif., said in a statement Friday that he had issued the subpoena to retired veteran diplomat Thomas Pickering to force him to appear at a deposition next week.

Pickering, who co-chaired the Benghazi Accountability Review Board with former Joint Chiefs of Staff chief Mike Mullen, has offered to testify before Issa’s committee in public.

But Issa said a closed-door meeting is needed first in order for the committee to fully understand how the review board conducted its investigation.